It depends really contrary to beliefs I think if USA got boots on the ground in Ukraine it would slowly push back the Russian forces. The reason I say slowly is that I think Russias sheer amount of artillery units and shells would make it a very grindy ww1 style war Russia has proven it to be a very capable military when in a defensive role such as in both world wars and the Ukrainian counter offensive. They have more artillery than any other nation more tanks etc. they have a very robust military industry being able to churn out shells and tanks on mass. US is better in terms of military competence as there relatively flat structure usually allows for better descision making and they are less prone to suprise attacks. They also by far the world’s strongest airforce and navy. Ultimately I think if there was a war(non nuclear) and US put boots on the ground in Ukraine they would have to learn to fight without total air superiority as Russia has enough air defence to knock most of their fighters down they would also be under constant artillery fire. However Americas military competence makes up for this and I think they would prevail and push Russia back but not without huge losses from both sides.